1. Field of the Invention
The present invention relates to a non-heat treated wire or bar steel for springs which possesses a high tensile strength of about 120-150 kgf/mm.sup.2 and exhibits good cold bending properties in its as-hot rolled state even though it does not undergo heat treatment (such as quenching and tempering) after hot rolling.
2. Description of the Related Art
Springs are divided into hard-drawn ones and heat-treated ones if they are made of a high-strength steel wire or bar with a tensile strength greater than 100 kgf/mm.sup.2. Hard-drawn springs are manufactured from intensively cold-drawn rod, typically piano wire made from eutectoid steel. Heat-treated springs are manufactured from rolled (or hot-rolled) and drawn rod by hot bending and ensuing heat treatment (quenching and tempering) or from previously heat-treated rod by cold bending.
Production of hard-drawn springs needs intense drawing and production of heat-treated springs needs heat treatment, as mentioned above, and they are only possible with large-scale facilities and heavy energy consumption and long processing time. If springs can be made from as-rolled rod without intense working and heat treatment, it would be possible to greatly save facilities and raw materials and to shorten the delivery period, and hence such a technology would be very useful.
The problem involved with as-rolled material is that it cannot be used as such (with a tensile strength in excess of 120 kgf/mm.sup.2) because it is too poor in toughness and ductility to undergo cold bending and it breaks springs with insufficient impact resistance.